I have been trying to understand Isru Chag. While there is a source in tehillim for the words and the gemara derives a notion of extra celebration, there seems to be little about the scope of the day.

What troubles me is that the codification of Isru Chag via the Ramo takes place in galut -- post-15th century. The last day of pesach would already have been logically obviated and maintained only as a derobonnon practice so the deriving of a day as an extension of the yom tov would already have been satisfied by the addition (and maintaining) of an entire second day of yom tov. Isru chag would still have value in Israel because otherwise, nothing would be "bound" to the end of the holiday. But outside of Israel, we are already "binding" a second day of yom tov.

We know that the last day of pesach is not "really" part of the biblically mandated holiday. So the derivation of the isru chag via a line in tehillim (written about the celebration PRE-yom tov sheini shel galuyos) to create another day seems unnecessary.

So shouldn't Isru Chag be a practice only for Ashkenazim in Israel? (I note no Isru Chag tag)